Conventionally, as a method for improving the corrosion resistance of aluminum and aluminum alloys (hereinafter also referred to as aluminum-based materials), an anodizing treatment has been employed by which a porous anodic oxide coating is formed on a surface of an aluminum-based material. In the porous layer of the anodic oxide coating, which is formed mainly by direct-current electrolysis, pores are regularly arranged in general, although this greatly depends on the electrolysis conditions. Hence, its porosity is one of the causes of degradation of the corrosion resistance. To improve the corrosion resistance of an anodic oxide coating, a sealing treatment for filling the pores or the like is conducted after the anodic oxide coating treatment.
As such an anodizing treatment, it is known that the film formation speed of an anodic oxide coating is improved by a method for an anodizing treatment on an aluminum-based material in which an aluminum-based material serving as a workpiece is immersed in a treatment bath, a direct current is applied thereto, and then an AC-DC superimposition current is applied in which a high-frequency current is superimposed on the direct current, and it is also known that this method may be applied to a piston head (PTL 1). In addition, a technology for improving the corrosion resistance is known which is not based on a sealing treatment but which is based on a dense anodic oxide coating in the vicinity of an aluminum-based material obtained by adjusting the electrolysis conditions in an anodizing treatment (PTL 2).